1. Field of the Invention
This invention relates to a sensor apparatus for sensing radiation from a scene. It also relates to a method of sensing such radiation.
2. Discussion of Prior Art
Sensor apparatus for sensing radiation from a scene are well known in the prior art. They find widespread application, for example, in portable consumer video and digital cameras and also in thermal imagers as employed by emergency services.
A typical apparatus incorporates a sensor comprising a two-dimensional array of elements, each with an associated signal processing circuit. Radiation from a scene is projected onto the array where each element responds via its associated processing circuit with an output Sk as given in equation [1]; the index k here is used to identify elements uniquely, i.e. Sk is the output from the kth element circuit. The output Sk includes unwanted artefacts which arise either from the scene itself or are generated within its associated element or processing circuit:
Sk=Ak(Rfk,Rpedk)+Bk+Nkxe2x80x83xe2x80x83Eq. 1
where
Sk=output generated from kth element via its associated processing circuit;
Ak=kth element responsivity function;
Rfk=feature information or scene contrast radiation from the scene received at the kth element;
Rpedk=background radiation from the scene received at the kth element;
Bk=offset signal generated within the kth element and its associated processing circuit; and
Nk=noise generated within the kth element and its associated processing circuit.
The outputs Sk from each element are combined to provide a sensor signal.
The artefacts generated within the sensor may arise, for example, from offset potentials generated within its processing circuits; these offsets can arise from circuit device semiconductor bandgaps or from transient charge injection effects when processing signals within the circuits.
The sensor including its associated circuits may be based on charge coupled devices (CCDS) or metal oxide semiconductor (MOS) devices. When MOS devices are employed, it is found that there is a noticeable variation in responsivity function Ak amongst the elements, namely the elements have differing responsibilities and give different outputs Sk in response to the same received radiation intensity. This variation is often larger than that of sensors incorporating charge-coupled devices (CCD). It has prevented widespread use of sensors incorporating MOS devices in consumer video cameras in preference to sensors incorporating CCDs despite a long-felt want to do so in order to benefit from the compatibility of MOS detection and processing circuitry; providing power supply and control signals for operating MOS devices tends to be less complex and less expensive compared to providing them for operating corresponding CCDs. The variation gives rise to fixed pattern noise (FPN) in the outputs Sk which results in the corresponding sensor signal depicting a speckled scene. Moreover, there is also a variation in offset signal Bk amongst the elements, and the responsivity function Ak and the offset Bk are often dependent upon sensor temperature. For sensors employed to respond to diminished radiation intensities, noise Nk generated within their elements and associated processing circuits often becomes a problem, in particular flicker noise contributing to Nk which has a noise spectral density which increases inversely relative to frequency.
When the sensor receives infra-red radiation from the scene, each output Sk is found to comprise an unwanted pedestal component, corresponding to a general background temperature of the scene, together with a desired feature component, namely corresponding to temperature variations within the scene. This is particularly pertinent where:
(i) the scene is at an ambient temperature of approximately 300K; and
(ii) the temperature variations within the scene giving rise to the feature component are less than 1K.
The pedestal component may often be a factor of one thousand or more larger than the feature component. This results in poor signal contrast which may render the temperature variations difficult to identify in the outputs Sk unless further signal processing is applied thereto.
The presence of the pedestal component imposes constraints and limitations on design and performance of a sensor apparatus for sensing emissions from a scene, especially infra-red emissions therefrom. The apparatus may, for example, need to incorporate analogue-to-digital conversion circuits providing a large dynamic range corresponding to 12-bits or more so that both the pedestal component and the feature component may each be resolved in data provided by the circuits. Moreover, the unwanted pedestal component may result in problems of saturation in sensor apparatus which analogue integrate photodetector signals in order to provide improved apparatus signal-to-noise performance.
A solution which addresses the problem of pedestal component described above is provided in a U.S. Pat. No. 5,155,348 which describes a read-out circuit for a sensor comprising a two-dimensional array of 128xc3x97128 photodetector elements responsive to infra-red radiation where each element is connected to its respective read-out circuit. In U.S. Pat. No. 515,534, the circuit is described as operating in calibration and measurement phases.
During the calibration phase, a calibration image is projected onto the elements. The image may correspond to a featureless calibration object of similar temperature to a scene to be viewed or a totally blurred featureless uniform image of the scene. Each element generates a signal in response to the calibration image and its respective circuit is arranged to store a calibration signal on a storage capacitor Cc incorporated within it corresponding to a signal generated by its respective element in response to the calibration image. This provides a correction for pedestal component across the array.
During the measurement phase, a focused measurement image of the scene is projected onto the array. A measurement signal generated at each element in response to the image has subtracted from it the calibration signal for that element to provide a difference signal. The difference signal is integrated within the circuit onto an integration capacitor Cs incorporated therein to provide an output signal. The circuits each produce a respective output signal which is multiplexed for generating a compound sensor signal.
This solution provides an advantage that the dynamic range of the compound sensor signal is reduced as a result of removing a pedestal component generated at each element. This eases dynamic range performance requirements of remote circuits receiving the sensor signal from the multiplexer, for example allowing use of analogue-to-digital converters of 8-bit resolution instead of 12-bit resolution.
A problem arises with the sensor described in U.S. Pat. No. 5,155,348 when scene contrast radiation Rfk is greatly diminished relative to the background radiation Rpedk, for example where the sensor is used to view a substantially uniform scene incorporating a distant faint object. Inaccuracy when subtracting the calibration signal from the measurement signal can result in the contrast radiation Rfk being masked by subtraction inaccuracies. One source of inaccuracy is transient charge injection within the element circuits which arises when the circuits are being switched between calibration to measurement phases. Transient charge injection can be conventionally reduced in a circuit as described in the U.S. Pat. No. 5,155,348 by reducing junction capacitances of MOS devices incorporated therein and increasing capacitance of its storage capacitor incorporated therein to provide a modified circuit. This results in a problem that the modified circuit occupies more space when fabricated in monolithic form and its speed of operation is degraded. Reduced speed of circuit operation can result in settling offsets arising when the element circuits are switched between calibration and measurement phases before potentials in the circuit have asymptotically stabilised. Moreover, it limits the rapidity with which image information can be output from the sensor.
The problems described above are reduced, according to the present invention, by incorporating an additional device into each element circuit. The device is arranged to inject a compensating charge into the circuit to compensate at least partially for transient charge injection arising when the circuit is switched from its calibration phase to its measurement phase. This alleviates the problems of reduced speed of operation and increased size described above for the modified circuit.
According to the present invention, a sensor apparatus is provided for generating a sensor signal corresponding to a filtered image of a scene, the apparatus incorporating:
(i) detecting means incorporating a plurality of detector elements and arranged to derive first and second element signals during first and second detection phases respectively; and
(ii) processing means associated with each element for deriving a difference signal from the element signals for use in generating the sensor signal;
characterised in that the processing means incorporates compensating means for counteracting inaccuracies introduced in response to switching the sensor apparatus between detection phases.
The invention provides the advantage that it reduces inaccuracies introduced when the sensor apparatus is switched between first and second phases, thereby improving accuracy of the apparatus when generating the sensor signal.
The sensor apparatus provides sensor signals in which faint distant objects are more identifiable compared to prior art apparatus. This is particularly important when the sensor apparatus is employed for detecting remote hazardous objects, for example in a maritime environment where early sensing for distant vessels representing a collision hazard is important.
The processing means may incorporate:
(i) storing means including a storage capacitor for recordal of a calibration signal therein derived from the first element signal during the first phase; and
(ii) current injecting means for injecting current onto the capacitor during the first phase and for providing a current during the second phase in response to the calibration signal recorded during the first phase for use in generating the difference signal, said injection means comprising a programmable current source incorporating self-cascoding MOS FETs.
This provides the advantage that the storing means can be integrated onto an integrated circuit. Moreover, the self cascoding MOS FETs provide the advantage of enhanced operating speed and accuracy.
The processing means may incorporate storing means including a storage capacitor for recordal of a calibration signal therein derived from the first element signal during the first phase; and the compensating means may incorporate a compensating capacitor comprising first and second electrodes, the first electrode connected to the storage capacitor for injecting a compensating charge thereonto and the second electrode arranged to be driven by a compensating signal for counteracting inaccuracies introduced into the storing means when the sensor apparatus is switched between detection phases. This enables the compensating means to be integrated in an integrated circuit.
The compensating signal may be in antiphase to a signal applied to the processing means for selecting the phases. This provides the advantage that an antiphase signal is relatively straightforward to generate and compensation is applied precisely when charge injection inaccuracies potentially arise.
The compensating capacitor may comprise a compensating MOS FET whose channel electrodes are shorted together to provide one of the electrodes of the capacitor and whose gate electrode is arranged to provide another of the electrodes of the capacitor. This provides the advantage of being a practical implementation of the compensating capacitor in an integrated circuit incorporating MOS devices for performing signal processing.
The storing means incorporates an enabling MOS FET for switching itself from the first phase where it stores its respective calibration signal into its storage capacitor to the second phase where it provides the calibration signal, and the compensating MOS FET incorporates a short channel so that its gate-channel capacitance is substantially half that of the enabling MOS FET. Substantially half is defined as being in the range of 25% to 75%. This provides the advantage that the compensating MOS FET provides a compensation capacitance of suitable value for providing effective compensation of inaccuracies arising from charge injection onto the storing means.
The elements and the processing means may be integrated together onto a substrate. This provides the advantage of a compact practical configuration for the sensor apparatus.
The processing means may incorporates interfacing means for interfacing from the processing means to its respective element and for presenting an input impedance to the element less than an equivalent internal impedance of the element, said interfacing means comprising a MOS FET configured in common gate configuration. This provides the advantage of being a practical circuit configuration for interfacing to the elements and providing reduced noise compared to prior art.
The sensor apparatus may incorporate projecting means for projecting first and second images onto the detecting means during the first and second phases respectively, where:
(i) at least one of the images is a projection of radiation from the scene; and
(ii) the images are of a differing degree of blurring to one another but neither being fully defocused and each retaining discernible spatial features,
thereby enabling the sensor apparatus to provide the sensor signal corresponding to a spatially filtered image of the scene.
Each degree of blurring may be such that radiation from a scene element focussable upon a single element becomes dispersed over a number of elements in the range of one element to 25% of the elements in the detecting means. This provides the advantage of a useful spatial filtration of the sensor signal.
The images may be blurred to a degree which is manually or automatically selectable. This provides the advantage that the degree of spatial filtration provided in the sensor apparatus is selectable to suit alternative uses of the sensor apparatus.
The second image may be blurred to a greater degree than the first image. This provides the advantage of image tone reversal in the sensor signal.
At least one of the first and second images may be a diffuse image. Using a diffuse image provides the advantage that it can be generated using more compact optical components than required for generating a defocused image.
The projecting means may incorporate a liquid crystal spatial light modulator configured to be controllable between a first state where it substantially transmits radiation unscattered and a second state where it transmits and scatters radiation from the scene to the detecting means for generating different degrees of blurring for the first and second images. This provides the advantage of a compact practical arrangement for implementing a diffuse image.
The liquid crystal spatial light modulator may be a polymer dispersed liquid crystal device (PDLC) configured to scatter radiation transmitted through it in one state and transmit light substantially unscattered through it in another state, the states being selectable in response to a control potential applied to the device. This provides the advantage of being a compact and inexpensive approach to generating diffuse images under electronic control without there being a need for mechanical moving parts.
Each element may comprise at least one of a cadmium-mercury-telluride photodiode, a photodiode with MOS readout, a phototransistor with MOS readout, a photogate with MOS readout and a photodiode with CCD readout. These provide the advantage of being sensitive photodetectors which collectively responsive over a wide spectrum of radiation wavelengths, for example wavelengths in a range of 10 xcexcm to 0.2 xcexcm.